Tattoos are permanent. They are generated by injecting small packets of the ink pigment into the lower levels of the skin, and these pigment capsules are too large for the body to naturally digest them. This means that the ink remains permanently in the skin. To remove the tattoo it is necessary to remove this pigment. Removal can be done in two main ways: Laser and Surgery.
LASER
 

The pigment can be removed by breaking up the little pigment capsules and allowing the pigment to be released so that the body can naturally absorb the remaining ink. This is how laser tattoo removal works.

Laser is very effective at treating the dark ink pigments in light coloured skin. It does this by a process called selective thermolysis. The black pigment absorbs the heat of the laser so a very localised heat is generated, which ruptures the small capsule containing the ink.
If the ink is of a lighter colour,
more similar to the colour of the skin, then the pigment in the skin and the pigment in the tattoo ink heat to a similar degree.

This means that it is more difficult to remove the coloured pigments such as yellow, red, and the paler greens, without heat-damaging the skin in the local area. Laser is therefore very effective at removing black tattoos, and less effective for multi-coloured areas.

Laser requires multiple treatments and even after successful laser treatment it is common to find a ghosting effect where a pale outline of the tattoo remains. This happens because the laser is removing the natural pigment from the skin, as well as the tattoo pigment, so you end up with a pale ghost of the original tattoo image.

  The coloured pigments can be treated with some specialist forms of laser, but again will be difficult to remove completely, and after a number of laser treatments it is quite common for the patient to end up with a pale pink and red coloured tattoo on a white ghost background